An imager of semi-conducting type is represented in FIG. 1. It comprises, generally, a detector material M, a first face of which comprises a large number of pixels P, possibly between a few thousand and a few million, generally disposed in matrix form and a second face comprising a large electrode E making it possible to polarize the detector. When the detector is irradiated by a radiation R generally of very low wavelength, each pixel P measures a signal S representing the energy yielded in the detector material by the interactions between the radiation and the detector material that have taken place in a zone of the detector facing this pixel.
In such detectors, the response of the pixels is not homogeneous. Stated otherwise, two pixels of the same detector subjected to identical radiation may produce two signals of different amplitude. One also speaks of spatial heterogeneity of response. This heterogeneity has several causes. Mention will be made notably of spatially variable detection characteristics of the detector material, it being possible for pixels to have differences in sensitivity with respect to one another. When this heterogeneity is stable over time, it is customarily corrected either by a simple gain correction and offset correction, or by a more complicated function of polynomial type.
A second significant cause of heterogeneity may be temporal instability or drift of the response of the detectors. The origin of this type of drift can stem from a strong irradiation which, through the appearance of a space charge, locally modifies the internal electric field of the material. Thus, the more the detectors are subjected to a significant quantity of integrated radiation, the more the spatial heterogeneity of response changes, doing so in a manner which is dependent on the irradiation history.
Moreover, in the course of one and the same acquisition, the sensitivity of the pixel, that is to say the signal delivered as a function of the incident radiation flux, can vary. There thus exist stable pixels whose sensitivity does not change with time and unstable pixels whose sensitivity changes with time. By way of example, FIG. 2 represents the temporal changes over a duration of several hundred seconds of the amplitude IS of the signal S of a pixel, measured as a number of impacts NI recorded during a given time period, or counting period or acquisition time. In the case of FIG. 2, the acquisition time equals 0.2 seconds, and the detector is irradiated for a duration of about 180 seconds. Each point of the curve corresponds to the number of strikes detected by the detector during a period of 2 ms. The number of strikes, or interactions detected, equals about 19400 at the start of irradiation, that is to say with a time t of close to 0 seconds, and decreases slowly with the irradiation time. After 3 minutes of irradiation, the number of strikes then equals fewer than 19000. This figure was obtained by exposing a pixel of CdTe, with dimensions 200 μm*200 μm and thickness 1.5 mm, with a beam of X rays, delivering a fluence rate, at the detector level, of 3×108 photons/s·mm2. This phenomenon is due to the appearance of space charge zones in a detector subjected to irradiation. These space charge zones lower the electric polarization field, the effect of which is a lowering of the sensitivity, hence a lesser number of interactions detected. It is a progressive phenomenon, giving rise to a spatially and temporally fluctuating polarization of the detector.
Consequently, the sensitivity of a matrix detector varies both spatially and temporally. Thus, the spatial heterogeneity of the response changes in the course of one and the same acquisition, and this phenomenon is also hard to predict. No solutions currently exist which make it possible to solve this problem in a satisfactory manner except by removing the trap levels responsible for the appearance of the space charge in the semi-conducting material. This makes it possible to minimize the problem, but zones of greater or lesser stability still remain. This problem is significant in so far as, in certain applications such as X-ray tomography, the stability criterion is paramount for avoiding artifacts in the images.